With the Internet rapidly replacing traditional telephone networks as the ubiquitous network infrastructure, there is ever-increasing consumer demand for greater bandwidth, which translates to a need for increased system performance. Coping with the continuing high growth rate of Internet traffic volume is a significantly challenging scalability problem. Fiber optics using Wavelength Division Multiplexing (WDM) offers the enormous capacity that the Internet requires to continue to grow at its present and projected future rates. In addition, the increasing agility of the latest Optical Layer Cross-Connects (OLXCs) offers the ability to dynamically change the optical layer connectivity on small time scales. OLXCs have the ability to convert the wavelength of any incoming channel to any outgoing wavelength (i.e. have wavelength conversion).
Internet Protocol (IP) network connectivity is more often being provided by optical circuits, including OC-48/192, for example. Thus, FIG. 1(a) is a schematic diagram showing the connectivity of IP layer 5 to an optical layer 10. FIG. 1(b) shows a more specific schematic diagram in which IP router 15 may be either hard-wired to Dense Wave Division Multiplexer (DWDM) 20 for transport, or it may be connected to OLXC 25.
There is an underlying conflict, however, between the typical datagram (connectionless) service that supports the best-effort data delivery of the Internet and virtual circuit (connection-based) service. This conflict is exacerbated in the world of optical networks, due to the fixed nature of the wavelengths available and the restoration of service in optical networks.
Optical networks are connection oriented and designed for fixed rate bit streaming with very low error rates. Whereas the Internet employs soft state where possible, the state of the optical infrastructure that is encoded in its OLXCs is hard and must be explicitly removed. The key elements in the success of the Internet have been its simplicity and the flexibility of the Internet service model, and therefore a significant challenge in leveraging the new optical capabilities to enhance the Internet and other services is to manage the optical resources efficiently, without sacrificing the simplicity and flexibility of the Internet.
In spite of most traffic and media types becoming internet protocol (IP) based, multiple-hop high-bandwidth optical connections referred to as lightpaths will continue to be of value. Aggregate loads between major metropolitan areas are rather stable, with most of the achievable statistical multiplexing already attained in the regional and collection (distribution) portion of the network. With electronic switching systems coping with substantial regional network volumes, this load can conveniently be assigned to point-to-point lightpaths that bypass intermediate backbone routers, reducing their load and reducing end-to-end delay and delay variation. Traffic engineering, i.e., load and quality management, is increasingly performed by adjusting connectivity and capacity between major backbone gateways on a relatively large time-scale, still small compared to the time-scale of provisioning.
This is both a primary function of, and a significant reason that, ATM or Multi-Protocol Label Switching (MPLS) is employed below the IP layer by most network operators. Agile, dynamically configurable OLXCs allow the use of the optical layer directly to implement these functions, avoiding having ATM or MPLS as intermediate layers in future networks. Lightpaths carrying transit traffic, or non-IP traffic, may remain a significant source of revenue for network operators for the foreseeable future. Whereas much of the transit capacity may carry IP traffic, operators leasing optical capacity may choose not to disclose this.
There are issues involving networks in general as they relate to where particular service and intelligence are provided. Functions previously provided by a SONET/SDH layer.
SONET (Synchronous Optical NETwork)/SDH (Synchronous Digital Hierachy) is an industry standard for broadband optical fiber communications. It provides universal optical interfaces at OC-N/STM-M rate. It also provides integrated OAM&P capabilities within each network element which enables fast protection/restoration. A good reference book is “Understanding SONET/SDH, Standards and Applications” by Ming-Chwan Chow, Andan Publisher, 1995.) interposed (not shown) above optical layer 10 must be distributed between IP layer 5 and Optical Layer 10 in the architecture of FIGS. 1(a) and 1(b), including the recovery of service after equipment failure.
Restoration may be provided by either the IP layer or the optical layer 10. The optical layer 10 is able to independently provide sub-second protection and/or restoration for link failures, that is when a fiber is cut, and is the most cost-effective solution therefore. However, when a router in the IP/Optical Layer architecture fails, the optical layer has no independent awareness of the router failure.
Thus, presently, it is the IP layer 5 that includes the necessary functionality for protecting against router failure. In addition, the IP layer 5 may include extra link capacity so that the quality of service may be preserved in the event of a router failure. As a result, it is then more cost-effective to use the extra link capacity to protect against link failure, and thus there is no incentive to utilize the protection/restoration function provided by the optical layer 10. Accordingly, IP network operators may choose a restoration strategy that depends solely upon the IP layer 5.
However IP layer restoration systems have some disadvantages. For instance, the failure of an unprotected link may result in a mean-time-to-repair in the range of four to ten hours although mean-time-to-repair for a router failure may be less than one hour. Still, the excessive amount of down-time due to a link failure may result in further router failures, which has the potential for significant network congestion.